1. Field of the Invention
The present invention is directed to nonobvious improvements in the preparation of tetrabutylammonium bis(fluorosulfonyl)imide, [Bu4N]+[(FSO2)2N]−, and related salts.
2. Background
Compounds containing bis(fluorosulfonyl)imide [(FSO2)2N]− are useful, for example, as Lewis acid catalysts, ion transport agents, in the fields of organic compound syntheses, electrolytes and the like.
Various methods for synthesizing bis(fluorosulfonyl)amine, and related compounds have been proposed (see, e.g., Ruff, Inorg. Chem. 4(10):1446 (1965); Ruff, Inorg. Synth. XI:138 (William, ed., McGraw-Hill Book Co., 1968); Vij et al., Coord. Chem. Rev. 158:413 (1997); Krumm et al., Inorg. Chem. 37:6295 (1998); Beran et al., Z. Anorg. Allg. Chem. 631:55 (2005); U.S. Pats. No. 8,377,406, 5,723,664; and 5,874,616; DE Patent No. 1 199 244). However, these methods may not be appropriate for industrial scale production either because they provide low yields, or require the formation of, e.g., dangerous intermediates and/or or corrosive/expensive starting materials.
A advances in obtaining [(FSO2)2N]− and its salts were disclosed by Morinaka (US2012/0028067 A1 and US2012/0070358 A1), incorporated by reference herein in their entirety, who treated a solution of SO2F2 in acetonitrile with ammonia gas, in the presence of an aprotic base, to obtain salts of [(FSO2)2N]− in high yield. As used herein “an aprotic base” has no labile hydrogen atoms. For example, diethylamine is a protic base and triethylamine is an aprotic base. By use of elevated pressure conditions, Morinaka was able to contain SO2F2 and thereby allow it to react with the ammonia at high concentration.
Those skilled in the art will recognize that Morinaka's processes use reactor pressures above atmospheric. The elevated pressure conditions described by Morinaka are problematic for commercial scale synthesis of [(FSO2)2N]− salts; since large scale synthesis would require large pressurized vessels. Large pressurized vessels for commercial scale synthesis can be cost-prohibitive, compared with lower cost vessels designed for use at atmospheric pressure and below. Additionally, there are increased safety issues which arise from the handling of SO2F2, at high pressure, which is highly toxic and completely undetectable by the senses or by common forms of measurement. Exposure of SO2F2 can be fatal to the operator.
Thus, there is a need for an improved synthesis of salts of [(FSO2)2N]−, which can be safely and economically scaled up for commercial application.
The present invention is based in part on the discovery that gas-phase reactor fouling occurs under the conditions of Morinaka's process, namely, at pressures above 0.01 MPa. Fouling is a problem which adds to the cost of labor, as the foulant must be cleared between runs. This extends well beyond a simple rinse, as any component whose surface comprises a part of the reactor head space must be disassembled. The foulant is a volatile, toxic, fluoride-containing slag. Even if the components are well cleansed, fouling can cause equipment failure through corrosion and abrasion of, e.g., stir bearings and seals. For industrial scale production of [(FSO2)2N]− salts, operation under conditions that do not foul the reactor, and minimize SO2F2 leakage hazards, are much preferred.